Disulfide bonds between Cys residues in adjacent strands of parallel beta-sheets are rare among proteins, which suggests that parallel beta-sheet structure is not stabilized by such disulfide cross-links. We report experimental results that show, surprisingly, that an interstrand disulfide bond can stabilize parallel beta-sheets formed by an autonomously folding peptide in aqueous solution. NMR analysis reveals that parallel beta-sheet structure is terminated beyond the disulfide bond, which causes deviation from the extended backbone conformation at one of the Cys residues.